Traditional materials (e.g., metals, plastics, ceramics, resins, concrete, etc.) do not always provide components with all the requisite properties sufficient for adequate performance under field service conditions. As is well known, one manner of modifying and/or enhancing the final properties of a component is to reinforce the primary material making up the component with one or more additional materials. One class of reinforced materials are matrix composites, which are generally formed from traditional materials (e.g., a matrix material) that include one or more discrete reinforcement constituents (e.g., a reinforcement material or component) distributed within a continuous phase of the matrix material. Such matrix composites exhibit functional and structural characteristics that depend upon, for example, the properties of the reinforcement constituent(s), the architectural shape and geometry of such constituent(s), and the properties of the interfaces between and among different constituents and the matrix material.
Composite materials typically include one or more different types of reinforcement materials. Particle reinforcement often includes non-metallic, and commonly ceramic, particles (e.g., SiC, Al2O3, etc.), but may include a variety of particles and materials that provide advantages or reinforcement for one or more properties of the matrix composite. Reinforcement of matrix material with fibers, including continuous-fibers, monofilament, and/or short-fibers is also known in the art. Generally, different types of matrix composites require or are typically associated with different primary processing routes/methods. Examples of different processes for forming matrix composites include, though are not limited to, in-situ reactive processes, diffusion bonding, blending and consolidation, vapor deposition and consolidation, liquid-state processing, stir casting/slurry casting, centrifugal casting, and infiltration processes involving infiltration of matrix material into porous preforms.
Some existing manufacturing and forming processes are designed to provide distributions of a reinforcement material within a matrix material. In some cases the reinforcement material may be distributed uniformly throughout an area, while in other cases the distribution may be non-uniform. In many cases, though, limitations with past techniques have led to less than desirable outcomes, resulting in a continuing search for forming processes, and corresponding composite materials, that exhibit desired structural and/or functional properties.
A brief overview of some processes that have been used to form composite materials will now be provided. In situ selective reinforcement methods involve placing and positioning a pre-cast reinforcement material member (sometimes referred to as a ‘preform’) into a near net-shape casting mold. Matrix material is then cast around the reinforcement member to form the composite. While the amount and/or density of pre-cast reinforcement material can be varied as desired, the constituent material of the reinforcement members does not become integrated (e.g., mixed or infiltrated) with the matrix material, except perhaps in a limited extent at the interfacial boundaries between the reinforcement member and the unreinforced matrix material. Therefore, such in situ methods are hindered by abrupt and problematic differential coefficients of thermal expansion (‘CTE’) between the matrix material and reinforcement member. Such abrupt transitions in CTE at the matrix-reinforcement interface boundaries can give rise to residual stress during the forming process (e.g., residual stress-concentration), and also manifest in stress fractures during thermal cycling of the reinforced components during service.
Another example of in situ selective reinforcement involves infiltration casting of matrix material into porous preforms positioned in near net-shape casting molds. The structure of the porous preform includes a reinforcement constituent, which may be uniform or non-uniform. One advantage of preform infiltration casting is that the method is relatively fast, thus resulting in a more integrated, infiltrated preform with substantially more contact area between the reinforcement and matrix materials. Even so, the materials still exhibit abrupt transitions in CTE at the interface/boundaries between the preform and the unreinforced matrix material that can create the stress problems noted above. Additionally, there are practical limits to the amount and density of reinforcement material that can be placed within a porous preform, because resistance to infiltration casting substantially increases at high reinforcement levels (e.g., beyond 15% to 20% material in the preform). In addition, the thickness and cross-sectional area of such preforms must be limited to allow complete infiltration prior to cooling of the matrix material.
Centrifugal casting techniques have been used to selectively reinforce composite materials by favorably placing or distributing reinforcement material to form gradient or layered distributions of the reinforcement material within the matrix material. While abrupt transitions in the coefficient of thermal expansion (‘CTE’) at the matrix-reinforcement interface boundaries can be reduced in centrifugal embodiments where continuous particle gradients are formed within the matrix material, such methods still suffer from differential CTE effects in cost-effective embodiments comprising layered reinforcement particles. Additionally, in centrifugal methods, the attainable variations of particle distributions are limited to bands or layers and/or continuous gradients, and if different reinforcement particle types having differing densities are simultaneously used, it may be impossible to get adequate coordinate (co-localized) particle gradient distributions for the divergent particle types, or to get the different particle types where they are needed, and in the desired pattern.
In further examples, another type of selective reinforcement involves the deposition or spraying (e.g., by low or high velocity spray techniques) of reinforcement particles onto the surface of near net-shape matrix material castings. One drawback of such methods for these applications is that the spray or deposition is superficial, because it is applied to the surface of solid matrix material castings, and does not substantially penetrate beyond the surface. Additionally, such superficial reinforcement coatings must generally be significantly machined prior to placing the reinforced casting into service. Moreover, absent resurfacing with more reinforcement, the effective service life of such castings is over once the superficial reinforcement layer is worn and/or otherwise degraded. Furthermore, in such superficial reinforcement applications, bonding and integration of the sprayed/deposited reinforcement with the matrix material is limited, even with the most optimal spray/deposition methods.
Gelcasting methods are another way to make functional gradient materials having preforms. In gelcasting, gradient reinforced preforms can be formed using gravitational or centrifugal forces to achieve a vertical composition gradient in molded slurries. The preforms may then be subsequently infiltrated. As with centrifugal casting embodiments, the attainable variations of particle distributions for preform gelcasting methods are limited to layers and/or continuous gradients. If different reinforcement particle types having differing densities are simultaneously desired/used, it may be impossible to get adequate coordinate (co-localized) particle gradient distributions for the divergent particle types, or to get the different particle types where they are needed, and in the desired pattern. Additionally, preforms made by such gelcasting methods are problematic because of excessive warpage and anisotropic shrinkage occurring during the sintering stage because of different sintering kinetics for the material components.
Accordingly, methods and processes exist to form composite materials, as well as composite materials having varying material densities such as reinforcement material gradients and other distributions, both non-uniform and uniform. As discussed above, though, limitations with past techniques have led to a continuing search for improved processes for forming composite materials, and corresponding composite materials and articles, exhibiting improved and/or desired structural and/or functional properties.